I'm stumped on this, need some ideas.
I installed the FMS MASS air kit on my truck, 95 F150.
ok, now with the kit, i'm getting codes 41,91, lean O2 sensors.
I have stock 19# injectors, adj. fuel presure reg, set to around 45 pounds.
GT40P heads (stock)
crane came 444221
107 lift 216 dur intake
117 lift 220 dur exhaust @.050

fuel pump good, changed O2 sensor, tried another MASS metter, any ideas?
I cant get any help from FMS on this, there anser was to send back the kit, with my cam, i need mass air.

thanks
chris

 

Does the truck actually seem to be running too lean? Keep in mind that with your mods it might create a situation that causes the readings to be outside of the normal parameters. If the truck actually is running lean you can fool the computer. Putting a resistor in line with the IAT sensor will make the truck think the air is cooler. This will richen the fuel mixture to compensate.
I'm not sure if the Fords use this sensor at all speeds or not. I have seen posts that indicate that some trucks only use the IAT sensor under certain conditions.